DESCRIPTION (Applicant's Description) Breast cancer is the most common cancer in American women and the second leading cause of cancer death in women. Natural killer cell-mediated immune responses may play a role in breast cancer control and prevention. Recent advances in the field of psychoneuroimmunology (PNI) suggest an integrated neuroendocrine-immune axis, such that stressors which activate central neural pathways also alter immune function. The stress-immune relationship is under explored in women in general, and especially in women at risk for breast cancer. Moreover, the clinical significance of stress-related changes in natural killer cell mediated responses in women with and without malignancy warrants further study. The premise which underlies this study is that stress-induced impairment of natural killer cell activity represents a potential mechanism whereby stress potentiates cancer progression and/or mediates sub-optimal response to therapy. Hence, the purpose of this pilot study is to determine if the experience of breast biopsy represents a human paradigm in which quantifiable stress is linked to impairment of natural killer cell activity. The aims of this study are to: (1 ) Determine the psycho-endocrine stress and natural killer cell profiles of women undergoing breast biopsy, and (2) Determine if low natural killer cell activity is related to a woman's psycho-endocrine stress profile and malignancy. The stress and natural killer cell profiles of women experiencing breast biopsy will be determined at four timepoints surrounding diagnostic breast biopsy. Women who exhibit stress and low natural killer cell activity pre-biopsy will have further natural killer cell studies performed to determine the basis of stress-induced alterations in natural killer cell activity. Women will be grouped based on biopsy results (malignant versus benign) in order to determine if a link exists between stress, natural killer cell activity, and malignancy. The results will lead to long term studies that can determine the psychological profile of women "at risk" for psychological distress (and possible altered natural killer cell activity) throughout the cancer trajectory. Early identification of such women can lead to the development of cost-effective psychological interventions designed to promote immunocompetence and hence, improve survival and quality of life of women with breast cancer.